PYROMETRY 417 FIG. 3.—Distance-read- shown in Fig. 3. A recording thermometer employs a mechanism for making a continuous record of temperatures on a suitable chart. Indicating and recording thermometers may be divided into three general classes, electrical, pressure, and bimetallic. Electrical thermometers will be discussed in later sub-sections ("Thermocouples," "Resistance Thermometers"). Pressure thermometers comprise a bulb containing a liquid or gas or both connected by means of capillary tubing to some form of pressure gage. Bimetallic thermometers utilize the turning force produced when a strip consisting of two metals having different coefficients of expansion and brazed to each other, is heated. Graphite-metal thermometers indicate temperatures as a result of the relatively large difference in thermal expansion of the two substances. Although of simple design, the accuracy and adaptability of these types of instrument has not been sufficiently investigated to allow a more detailed discussion of them here. Pressure thermometers may be sub-divided into vapor-pressure, liquid-filled and gas-filled instruments. From outward appearance vapor-pressure thermometers may be distinguished from liquid- or gas-filled instruments by the form of the tempp™*1™ enoi** v«!-»/•«._ pressure thermometers have an the length of the intervals for a, ^ increasing temperature. Liquid- an^ _ ... . __A___„ divided scales. The bulbs, capillary tubing and form of pressure gage may be identical in outward appearance. The pressure under which a vapor-pressure thermometer operates is a function of the temperature of the bulb only. The liquid selected to cover a given temperature range should have sufficient pressure at the lower temperatures to insure a readable temperature scale in this region. The lower the initial pressure the more constricted the lower part of the temperature scale will be as compared with the upper part of the scale where the range of pressure for a given change in temperature is much greater. The maximum temperature to which the liquid used in a vapor-pressure thermometer will be heated should not exceed its critical temperature. In general the temperature range for a given liquid is included between its boiling point at atmospheric pressure and its critical temperature. The initial pressure and the pressure range of a liquid- or gas-filled thermometer will vary according to the strength of the gage used and the temperature range. Since the increase in pressure with temperature is approximately the same over the entire scale, the temperature scale is evenly divided. The pressures in liquid- or gas-filled thermometers are made relatively high to insure a rugged gage mechanism. The following table gives some characteristics of vapor-pressure and liquid- or gas-filled industrial thermometers. The action of the vapor-pressure thermometer depends upon the fact that the pressure inside the thermometer is determined solely by the temperature of the free surface of the liquid. It follows therefore that the thermometer must be so constructed that one free surface is always in the bulb. If this condition is fulfilled the reading of the instrument will not be sensibly affected by changes in the temperature of the gage and capillary.